This invention relates to the production of metal chlorides such as AlCl.sub.3. More particularly, this invention relates to a fractional distillation process for separating metal chlorides from a mixture containing metal chlorides including AlCl.sub.3 and FeCl.sub.3.
As used herein, in accordance with common trade practice, the term "metal chloride" includes silicon tetrachloride (SiCl.sub.4).
In a process for the production of metal chlorides by the chlorination of a material containing metal oxides, a mixture of chlorides and non-condensable gases will be obtained. For example, in a process for the production of AlCl.sub.3 by the chlorination of kaolin clay in the presence of a carbonaceous reducing agent, a mixture of metal chlorides, including AlCl.sub.3, FeCl.sub.3, SiCl.sub.4 and titanium tetrachloride (TiCl.sub.4), will be obtained. Also obtained will be various non-condensable gases (at the temperatures and pressures encountered in the chlorination system) such as nitrogen (N.sub.2), carbon monoxide (CO), carbon dioxide (CO.sub.2) and chlorine (Cl.sub.2), chlorides such as hydrogen chloride (HCl) and phosgene (COCl.sub.2), and small amounts of other metal chlorides such as calcium chloride (CaCl.sub.2) and sodium chloride (NaCl).
Various processes for the separation of metal chlorides from a mixture of chlorides and non-condensable gases are known. Several of these processes involve the selective condensation or fractional distillation of particular metal chlorides from the mixture. Thus, for example, the commonly owned, copending application of Johnson, Stewart, Tsao and Wohleber, Ser. No. 40,945, entitled "Purification of Aluminum Chloride", describes a three-stage fractional distillation process for separating AlCl.sub.3 from a mixture of chlorides and non-condensable gases. According to this process, the mixture is introduced into a first distillation column, which is operated at temperatures sufficient to separate SiCl.sub.4 from the mixture. The undistilled chlorides from the first column are then introduced into a second distillation column, which is operated at temperatures sufficient to separate TiCl.sub.4 from the remainder of the chlorides. A portion of the TiCl.sub.4 separated by the second column is added to the first column or upstream thereof to prevent precipitation of AlCl.sub.3 and to dissolve any solid AlCl.sub.3 condensing in the first column. The undistilled chlorides from the second column are then introduced into a third column to distill AlCl.sub.3 from the remaining chlorides (which include FeCl.sub.3).
An alternative three-stage fractional distillation process is also disclosed in Application Ser. No. 40,945. In this alternative process, a mixture of chlorides and non-condensable gases is introduced into a first distillation column, which is operated so as to separate SiCl.sub.4 and TiCl.sub.4 from the mixture. The mixture of SiCl.sub.4 and TiCl.sub.4 from the first column is passed to a second distillation column, which is operated so as to separate the SiCl.sub.4 from the TiCl.sub.4. A portion of the TiCl.sub.4 from this second column is returned to the first column to inhibit the accumulation of solid AlCl.sub.3 therein. The undistilled chlorides from the first column are introduced into a third distillation column, which is operated so as to separate AlCl.sub.3 from the other chlorides.
U.S. Pat. No. 2,387,228 to Arnold describes a two-stage fractional distillation process for the separation of AlCl.sub.3 from a mixture containing AlCl.sub.3, FeCl.sub.3, SiCl.sub.4 and TiCl.sub.4. According to this process, the mixture is introduced into a first distillation column, which is operated at temperatures and pressures sufficient to separate a mixture of AlCl.sub.3 and FeCl.sub.3 in the liquid state from the remainder of the chlorides. This liquid mixture is then introduced into a second distillation column, which is operated at temperatures and pressures sufficient to separate AlCl.sub.3 in the gaseous state from the mixture of liquids.
Other known processes for the fractional distillation or selective condensation of particular metal chlorides from a mixture of chlorides include that of U.S. Pat. No. 3,436,211 of Dewing and the process of U.S. Pat. No. 3,786,135 to King et al. The Dewing process operates to remove calcium chloride (CaCl.sub.2) and magnesium chloride (MgCl.sub.2) from a gaseous mixture containing these chlorides and AlCl.sub.3, and the King et al. process operates to selectively condense sodium aluminum chloride (NaAlCl.sub.4) from the gaseous effluent derived from the chlorination of alumina which is contaminated with sodium.
Unfortunately, however, problems have arisen in many instances where a fractional distillation or selective condensation procedure has been utilized to separate a particular constituent from a mixture of metal chlorides. Such problems may arise because the range of temperatures found in distillation columns operated at convenient pressures for the separation of particular metal chlorides from the mixture may include temperatures at which undesirable condensation of other metal chlorides takes place. Thus, for example, as described in the aforementioned application of Johnson et al., Ser. No. 40,945, now abandoned, the condensation of AlCl.sub.3 from a gas to a solid may interfere with the operation of a distillation column at temperatures and pressures sufficient to separate SiCl.sub.4 from a mixture containing AlCl.sub.3, FeCl.sub.3, SiCl.sub.4 and TiCl.sub.4. Such a column may be operated at a convenient pressure with a temperature at the bottom of the column higher than the melting point of the mixture at that pressure, and a temperature at the top of the column at or above the boiling point of SiCl.sub.4 at that pressure. Thus, the range of temperatures within the column may include the sublimation temperature of AlCl.sub.3 ; consequently, appreciable amounts of AlCl.sub.3 may condense to a solid within the column and thereby clog or plug it. This plugging problem is solved by the method of Johnson et al., which provides for the maintenance of an amount of TiCl.sub.4 sufficient to prevent solidification of AlCl.sub.3 in any column which is operated to separate at least one metal chloride including SiCl.sub.4 from a mixture containing AlCl.sub.3 and other chlorides and which is operated at temperatures which include the sublimation point of AlCl.sub.3 at the operating pressure of the column.
A similar problem is discussed in U.S. Pat. No. 2,718,279 of Kraus, which describes a fractional condensation process involving the separation of FeCl.sub.3 from a mixture of gases including FeCl.sub.3 and TiCl.sub.4. According to this reference, fractional condensation of such a mixture is hindered by the tendency of FeCl.sub.3 to condense or precipitate on the interior surfaces of conventional condensing apparatus which are operated at convenient temperatures and pressures for the separation of TiCl.sub.4 from the mixture. According to the method of Kraus, this problem is overcome by the washing of the FeCl.sub.3 from the gaseous mixture with a continuous flow of liquid TiCl.sub.4.
U.S. Pat. No. 2,870,869 to Mahler describes a problem which has accompanied the separation of TiCl.sub.4 from a gaseous mixture containing TiCl.sub.4 and other metal chlorides, including AlCl.sub.3 and FeCl.sub.3. According to this reference, the condensation of TiCl.sub.4 from such a mixture may be accompanied by the formation of a considerable amount of finely divided solid particles of AlCl.sub.3 and FeCl.sub.3. According to the process of Mahler, this problem is solved by the provision of a chamber containing a slurry of solid AlCl.sub.3 and FeCl.sub.3 suspended in liquid TiCl.sub.4. This slurry is maintained at a temperature close to but below the dew point of TiCl.sub.4 and is agitated within the chamber so as to provide a turbulent shower therein. The gaseous mixture is then passed through the chamber and the gaseous AlCl.sub.3 and FeCl.sub.3 in the mixture are reportedly washed therefrom by contact with the turbulent shower.
Thus, it appears that considerable difficulties have been encountered in fractional distillation procedures for separating particular metal chlorides from a mixture containing various metal chlorides including AlCl.sub.3 and FeCl.sub.3. These problems have arisen because of the tendency of AlCl.sub.3 and FeCl.sub.3 to solidify in fractional distillation columns which are operated at temperatures and pressures which are suitable for the separation of particular metal chlorides from such mixtures. Various techniques aimed at removal or dissolution of the AlCl.sub.3 and/or FeCl.sub.3 by the addition of a wash medium or solvent to the mixture have been attempted. However, such procedures are often cumbersome and complicated and may involve the addition of specialized apparatus and piping.
It would be desirable if a fractional distillation process could be developed for separating particular metal chlorides from a mixture containing AlCl.sub.3 and FeCl.sub.3 which avoids the problem of clogging of distillation columns caused by the undesirable solidification or condensation of AlCl.sub.3 and FeCl.sub.3 and which further avoids the complicated washing techniques of known processes.